The invention relates to power amplifiers. In particular, the invention relates to a switchable amplifier operating between at least two power levels which has reduced switching distortion, fast response time and improved power handling.
A class-G power amplifier disclosed in Sampei, U.S. Pat. No. 3,961,280, switches between two levels of rail voltages (two positive and two negative rails) to provide a more efficient and cooler running design than single rail voltage approaches (with one positive and one negative rail). At idle and at relatively low signal conditions, the class-G power amplifier uses the low supply rails. As signal conditions increase, i.e., when the output swing exceeds the lower supply rail voltage, the amplifier automatically switches to the higher supply rails for that portion of the output swing that exceeds the lower rail voltage. This approach improves the overall efficiency of the design because for a large portion of the signal conditions, the amplifier uses the low voltage rails which are typically one-half the voltage of the high voltage rails. As a result, the input power, and ultimately the power dissipation is less than that of the conventional class-AB approach. At lower power levels, power dissipation is approximately one-half of the class-AB value. Thus, with the same output power level, a switchable class-G design can use smaller heat sinks and power transformers than a comparable class-AB design.
Most known class-G amplifier designs, however, have severe performance limitations at high frequencies. Typical slew (switching) rate values are low, and the total harmonic distortion (THD) is high due to device switching limitations. In known arrangements, for example, switching is accomplished by a diode and a pair of power transistors. The saturation and switching characteristics of such devices determine the transient response of the output signal at the rail transition and limits the speed at which the output can slew from rail to rail.
FIG. 3 shows the basic circuit of a known class-G amplifier, similar to Sampei above. In the arrangement, two different voltage supplies V.sub.L and V.sub.H representative of the respective low and high power supplies (rails) are coupled to a pair of corresponding amplifiers Q.sub.L and Q.sub.H connected in series, as shown, with the emitter of Q.sub.H coupled to the collector of Q.sub.L. A switching diode D.sub.S is connected between the series connection of the amplifiers Q.sub.H and Q.sub.L, as shown, and to the lower voltage supply V.sub.L. A similar arrangement is provided on the complementary or opposite polarity side using respective negative low and high voltages, -V.sub.L and -V.sub.H and amplifiers Q.sub.L ' and Q.sub.H ', which are devices of opposite conductivity type as illustrated.
In the arrangement illustrated in FIG. 3, the device Q.sub.H or Q.sub.H ' is referred to as the upper device and the device Q.sub.L or Q.sub.L ' is referred to as the lower device. When the input signal V.sub.in is less than the lower rail voltage V.sub.L, the upper device Q.sub.H is cut off, and the lower device Q.sub.L supplies output current via D.sub.S which is forward biased. When the input signal exceeds the lower voltage V.sub.L the upper device Q.sub.H is turned on. When this happens, the switching diode D.sub.S is reverse biased and cuts off the current from the lower voltage V.sub.L for that portion of the output signal swing that exceeds V.sub.L. The diodes D.sub.L and D.sub.H which are in the respective base circuits of the lower and upper devices, establish a voltage relationship between the devices so that the lower device Q.sub.L is prevented from going into saturation.
Attempts have been made to improve the efficiency and performance of class-G operation. For example, Dijkmans et al., U.S. Pat. No. 4,706,035, shows two level output (two positive and two negative) class-G amplifier with a bootstrap capacitor for driving the output up to the second supply voltage. A switching diode is in the output circuit of the low voltage source. A series diode circuit is provided between the bases of the output devices. Although the bootstrap circuit improves output swing, the upper output device still becomes saturated, thereby reducing the system performance.
In class-G operation switching distortion will occur twice each half-cycle as the input signal V.sub.in increases above or decreases below the lower voltage V.sub.L. The distortion caused in conventional class-G operation is due to switching delays occurring at such transitions.